1. Field of the Invention
This invention relates to a high speed biCMOS gate driver for use in conjunction with power MOSFETs.
2. Brief Description of the Prior Art
In order to minimize switching losses in switched-mode power converters, the power MOSFET transistors must be switched on and off very rapidly. The gate drive signal must respond or slew at nearly 10.sup.9 volts/second in order to achieve state of the art performance. Since the equivalent gate capacitance of a power FET is typically several hundred picofarads, peak currents of 500 milliamperes to several amperes are required to generate the necessary slew rates. These currents are driven for intervals of less than 50 nanoseconds with very low duty cycles. To obtain high efficiency, the gate driver circuit should consume little or no standby current. Driver circuits for this type of application have been very difficult to design. Existing implementations use either pure bipolar or pure CMOS outputs. Pure bipolar circuits are plagued by shoot-through currents which are currents that occur when both the upper and lower switches in a totem-pole output are on at the same time due to overlaps in the switching waveforms and require excessive standby supply current. Shoot-through currents are very difficult to eliminate in bipolar gate drivers. CMOS drivers have poor current drive capability relative to the die area they consume.
Some drive configurations, particularly the bootstrap configuration widely used to drive power NMOS transistors in a high-side-drive (HSD) configuration, subject the gate lead to voltages one diode drop below ground. The standard BiCMOS driver has insufficient immunity to substrate injection to handle this application, but by use of appropriately placed diodes and by reconnecting the power supplies in a novel manner, a BiCMOS driver with superior immunity to substrate injection can be formed, substrate injection being the injection of minority carriers from a forward biased N-type diffusion to the P-type substrate. A typical prior art driver with bootstrap configuration is shown on page 271 of Analog Circuit Design, Edited by Rudy J. Van DePlassche et al., Kluwer Academic Publishers, 1995. Prior art biCMOS gate driver circuits exist which overcome the limitation of pure CMOS or pure bipolar gate drivers.